Manganese-activated complex fluoride phosphors such as K2TiF6:Mn4+, K2SiF6:Mn4+ and K2GeF6:Mn4+ have poor reliability (stability), resulting in reduction of brightness and shift in CIE coordinates over prolonged periods of operation (typically, 100's of hours of use). Clearly, there is a need for coatings which are effective at protecting such phosphor particles from moisture and oxygen thereby improving the brightness and reducing the shift in CIE coordinates over prolonged use so as to provide a commercially useful phosphor.